It is always a delicate balance. The issue with half-doses is that if a small portion of the population has some resistance to the medication, a half dose may not be enough to kill them off because of their resistance. Then, all you end up doing is selectively breed a strain of ich resistant to the medication.
This is the same reason why we have very resistant strains of bacteria today -- casual overuse of antibacterials, and incomplete use of antibacterials too.
But, of course, you don't want to kill the fish either. So, it there really isn't a clear cut answer. The dosing suggested by the bottle is a level tested to be able to kill most strains of ich and leave fish relatively unharmed. I'd use the full dose whenever possible.
